With the increasing number of different ringtones produced by the telephone manufacturers today, the ability to detect different ringtones in an automatic off-hook switching device becomes more and more important.
WO 99/46910 A1 describes a desk phone coupled to an interface device comprising a ringtone detector and an off-hook detector for decoupling the desk phone handset when a call is accepted on the headset. The interface device has three different ringtone learning methods which can be used to detect and learn the characteristics of a new ringtone detected on the telephone line.
JP 7240773 A describes a different telephone apparatus comprising a processor having a learning procedure which is able to detect, learn and store the characteristics of a ring cadence on the telephone line network.
U.S. Pat. No. 4,540,855 A describes a telephone apparatus containing an adaptive digital filter capable of adjusting the bandpass filter parameters to the audio information transmitted on the telephone line. The digital filter contains a phase loop lock which locks on to the periodic ringtone signal, which is lost when a call is accepted.
These devices tend to be bulky and require the desk phone to have an interface connection that enables a wired connection between the desk phone and the device.
US 2004/0131174 A1 describes a desk phone coupled to a hook switch device comprising a ringtone detector that automatically hooks off the handset when a call is accepted. The hook switch device then relays the call to a wireless headset. The user places the sensor at a position from which the strongest magnetic field is sensed during a calibration process.
None of these devices describe a ringtone detector capable of detecting and learning new ringtones with an absolute minimum of user interaction, and an automatic off-hook switch device which is compact and easy to connect and install.